MAGNETIC CHARACTERS OF METALS, &c. 655 
magnetic piece of brass will cause it to become 
magnetic. An instance of this kind I have never 
yet met with : but I have found that when an un- 
hammered piece has been so slightly magnetic as 
to have that character but just discernible, ham- 
mering it so as to compress its two sides closer 
together, gives it an increased magnetic action ; 
which may possibly be a consequence of bringing 
the whole of its magnetic particles more com- 
pletely within the range of the testing magnetic 
influence; and I am inclined to believe that, had 
Cavallo’s test been more powerful than a magnetic 
needle, he would have found that those pieces 
whose magnetism he thought was due to com- 
pression alone, were slightly magnetic previously. 
Still, however, there is a possibility that magnet- 
ism might be detected in compressed brass, in 
which that power is too feeble to be detected 
whilst the metal is in an uncompressed state, 
even by powerful magnetic tests. 
41. In addition to the advice given by Cavallo, 
respecting the necessary caution in employing 
brass in the construction of compass boxes (note 
to 6,) I should advise the makers of those useful 
instruments to test every piece of brass, intended 
to be employed in their construction, by a powerful 
